49ers’ Kittle, Breida have separated shoulders, but may play in Week 1

San Francisco 49ers tight end George Kittle (85) cannot catch a pass between Dallas Cowboys defensive back Jeff Heath, foreground, and linebacker Jaylon Smith, rear, during the first half of an NFL preseason football game in Santa Clara, Calif., Thursday, Aug. 9, 2018. (AP Photo/Josie Lepe)

Photo: Josie Lepe / Associated Press

All six 49ers who exited San Francisco’s preseason opener Thursday are expected to be available for the team’s Sept. 9 season opener in Minnesota.

On Friday, head coach Kyle Shanahan was optimistic that the injuries sustained by four starters and two key backups in a 24-21 win over the Cowboys won’t be long-term.

“Yes,” Shanahan said, “we’re hoping they’ll all be available Week 1.”

The most serious injuries were sustained by tight end George Kittle and running back Matt Breida, who both have separated shoulders and will miss the final three preseason games. Neither shoulder injury involved structural damage. Inside linebacker Malcolm Smith (hamstring) could return for the third preseason game at Indianapolis on Aug. 25, outside linebacker Eli Harold (knee contusion) is day-to-day and defensive tackle Solomon Thomas and offensive tackle Garry Gilliam are in the concussion protocol.

In other injury news, Shanahan said he would be “very surprised” if cornerback Richard Sherman (hamstring) plays in the second preseason game at Houston on Aug. 18. He said it was possible right guard Jonathan Cooper (knee), who has been eased back into practice after undergoing offseason surgery, could make his preseason debut against the Texans.

In addition, right guard Joshua Garnett (knee), who hasn’t practiced since July 28, could return to practice next week. At one point during training camp, Shanahan expressed surprise that Garnett wasn’t practicing. On Friday, he said Garnett had swelling in the same knee in which he had surgery last year and had received an anti-inflammatory shot.

“There’s not an exact injury,” Shanahan said. “I think you could ask him exactly about that.”

Strong debut: Shanahan said he was “really happy” with rookie right tackle Mike McGlinchey after the No. 9 overall pick played into the second quarter in his NFL debut, logging 23 snaps.

“It was very easy to see that the game is not too big for him,” Shanahan said. “He’s very comfortable out there; looks like the player he looked like in college when we studied him.”

McGlinchey and right guard Mike Person (40 snaps) were the only starting offensive linemen to play more than one series. Person, 30, started 14 games at center for the Falcons in 2015 when Shanahan was Atlanta’s offensive coordinator.

Person, who signed in May, has become a candidate to start while Garnett and Cooper have been sidelined.

“I’d say Mike’s been the same guy that we knew of, if not a little bit better,” Shanahan said. “… I think he’s gained some confidence and I thought he looked similar to how he always looks in the game. I thought he did a good job.”

Planning ahead: After bulling his way for a 1-yard touchdown run in the second quarter, running back Joe Williams said he would look into an alternate way of ending up in the end zone.

“I’m going to work on diving over the top this week,” Williams said, smiling.

Williams acknowledged he probably had other work to do after he had 27 yards on 11 carries. It was his first game since the 2017 fourth-round pick was put on injured reserve before last year’s season opener because of an ankle injury.

“There were probably some things I missed here and there,” Williams said. “But for it to be the first game going against another defense, I felt like I did a decent job. I’ll see what it looks like on the film.”

Williams could be fighting for a roster spot as the No. 3 running back with Raheem Mostert, who has more special-teams value than Williams. Mostert had 57 yards on eight carries, but lost a fumble.

First flag: Inside linebacker Elijah Lee became the first member of the 49ers to be flagged for lowering his helmet, a rule the NFL is implementing this season.

In the third quarter, Lee was penalized for lowering his helmet and hitting Cowboys running back Bo Scarbrough around the torso at the end of an 11-yard reception. Players at all positions will be flagged this season for lowering their helmets to initiate contact.

“They are trying to make the game safe so, at all costs, you have to keep your head out of it,” Lee said. “… In the moment, you’re like, ‘Oh, man, I got the flag,’ but at the end of the day you have to understand it’s in your best interest.”

Eric Branch has worked at the San Francisco Chronicle since 2011 as the 49ers beat writer. Before that, he covered the 49ers for the Santa Rosa Press Democrat in 2010. Since he began his career in journalism in 1997 in Logansport, Ind., he’s covered events ranging from archery tournaments to the NCAA Men’s Basketball Tournament.